High-potential-side driver switches, i.e. driver switches disposed on the side of high potential of the supply voltage, in the form of MOS transistors require a charge pump, in particular when DMOS transistors (MOS transistors with double diffusion) are concerned, in order to be able to bring them to an on-state with low voltage drop. Monolithically integrated charge pumps require a chip area that depends on the necessary charge transfer rate. This means, the more charging current a charge pump must be able to deliver, the more chip area is occupied by it. Integrated charge pumps which can deliver so much charging current as is required for rapidly switching high-potential-side MOS power transistors, would need a considerable part of the entire chip area. For, in order to rapidly drive a MOS power transistor to the conducting state, the gate thereof must be fed temporarily with a quite high charging current which is higher by orders of magnitude than the charging current necessary for keeping a conducting MOS transistor in the conducting state once the gate thereof has been charged.
Since charge pumps capable of making available such charging currents cannot be integrated reasonably, it is conventional to use additional charge storage means, in particular in the form of bootstrap capacitors in which charge is stored which, during the times of high charging current requirements, namely during the switching-on edges, deliver charge stored therein to the gate of the MOS transistor to be switched. To be able to fulfill this function, bootstrap capacitors must have such high capacitances that they cannot be integrated monolithically. Bootstrap capacitors thus are formed by external capacitors which must be connected to the integrated circuit via connecting areas of their own. This does not only increase the number of external circuit components but also the number of necessary connecting legs of the integrated circuit.
For slowly switching circuits (with relatively low clock frequencies of some hundred Hz) one usually uses only a charge pump. If one comes to higher switching frequencies, the average current of the charge pump is EQU I=f*C.sub.GD *(V.sub.S +V.sub.CP)+I.sub.0 ( 1)
In this formula
I is the average supply current of the charge pump PA1 f is the switching frequency of the high-potential-side driver PA1 C.sub.GD =gate-drain capacitance of DMOS transistor PA1 VS=supply voltage of circuit PA1 V.sub.CP =pumping voltage of charge pump PA1 I.sub.0 =bias current of driver for high-potential-side DMOS transistor to be switched.
As already mentioned, fast switching circuits usually use a bootstrap circuit to reduce the load of the charge pump. The load to be fed to the gate of the transistor in order to switch the same on rapidly is delivered by the bootstrap circuit. The load acting on the charge pump is reduced to the static current consumption of the driver circuit, namely EQU I=I.sub.0 ( 2)
A conventional circuitry which is shown in FIG. 12 contains an N-channel MOS switching transistor M connected between a high-potential-side voltage supply terminal V.sub.S of a supply voltage source and an output terminal OUT and serving to switch a load LOAD connected between OUT and a ground terminal GND. A gate terminal G of MOS transistor M is connected to the output of a driver stage DR which is fed with a digital driver control signal via a control input SE. Driver stage DR is located between a circuit node K and a ground terminal GND and switches the gate G of MOS transistor M, in accordance with the respective potential value of the digital driver control signal, to the potential of circuit node K or to the ground potential of ground terminal GND. Circuit node K constitutes the pumping voltage output of a charge pump which has a pumping capacitor Cp, with one electrode thereof being fed with a rectangular pumping pulse sequence and the other electrode thereof being connected on the one hand via a first diode D1 to the supply voltage terminal Vs and on the other hand via a second diode D2 to circuit node K. Between K and Vs, there are disposed a third diode D3 and a fourth diode D4 in series connection. The cathodes of all four diodes D1 to D4 are directed towards circuit node A. Between a connecting point P between the two diodes D3 and D4 and OUT there is provided a bootstrap capacitor Cb.
To be adapted to be charged as quickly as possible to the desired pumping voltage, pumping capacitor Cp has a relatively small capacitance. This makes said capacitor indeed suitable for monolithic integration, but it has the effect that the charge pump cannot supply a sufficient charging current to the gate G of transistor M in order to rapidly switch the same to the conducting state. To overcome this problem, bootstrap capacitor Cb is provided which, outside the switching-on edges of transistor M, is charged from the supply voltage terminal V.sub.S via diode D4 and, at the times of occurrence of the switching-on edges of M, delivers the charge stored therein via diode D2 to the gate G of M and thus makes available the high charge which the gate G of transistor M requires for rapid switching on. To be able to fulfill this function, bootstrap capacitor Cb requires a considerably higher capacitance than pumping capacitor Cp. A capacitance as is necessary for Cb cannot be integrated monolithically with reasonable expenditure. This is why an external capacitor must be used as bootstrap capacitor Cb, which has to be connected via an additional connecting area B to the integrated driver circuit. In case of an integrated circuit having several high-potential-side MOS driver transistors, such as a full bridge circuit for a multipole stepping motor, this concept requires a bootstrap capacitor and a terminating area therefor for each high-potential-side MOS transistor of the integrated circuit.
If this known circuit arrangement with charge pump did not have a bootstrap capacitor, the gate G of MOS transistor M to be switched, during the times in which a switching-on control pulse is supplied via control input SE, would be fed completely from the charge pump.
According to equation (1), an average current to be delivered by the charge pump results as follows: EQU Im=f*C*.DELTA.U. (3)
The capacitance to be charged with this current is EQU C=C.sub.GD +C.sub.GS. (4)
Since for C.sub.GD the entire voltage swing of 0 to V.sub.S +V.sub.CP becomes effective, for C.sub.GS however only the switching-on threshold voltage Vth of transistor M, C.sub.GS may be neglected. That is to say, the capacitance to be charged by charge pump is EQU C.apprxeq.C.sub.GD. (5)
Due to the fact that the charge pump has to accommodate the entire voltage swing EQU .DELTA.U=V.sub.S +V.sub.CP ( 6)
a correspondingly high charge pump current results according to equation (3).